A so-called "electronic" sensor in an automobile vehicle is a transducer which tests a rotating mechanical member of the vehicle, the speed of rotation of this mechanical member being representative of the number of wheel revolutions made by the vehicle, and the generally alternating electric signal furnished by this transducer corresponding to electric pulses representative of this number of wheel revolutions.
Conventionally, this rotating mechanical member is most often one of the gears of the gear box.
It is possible to avoid fraud on the taximeter or chronotachograph which receives and processes these electric pulses by rendering all the circuit included between the output of the sensor and the taximeter or chronotachograph, impregnable by means of a mechanical shielding.
Such mechanical shielding prevents, in particular, the taxi--or truck driver from inserting two in the two respective signal output wires of the sensor so as to be able to connect thereto a pulse generator in parallel on said output of the sensor. In this method of fraud, the defrauder often chooses a pulse generator whose output impedance is much lower than that of the sensor, and the pulses delivered by this generator either smother the pulses delivered by the sensor and therefore replace them, or are added thereto.
The assembly of the sensor itself may advantageously be mechanically shielded, and therefore rendered impregnable, by means of a bell which covers the sensor and which is fixed and sealed with leads on the gear box.
However, modern vehicles are now equipped more and more often with a so-called A.B.S. braking system which uses an electronic sensor for each vehicle wheel. Generally, these vehicles are equipped with disc brakes on the four wheels, and consequently each electronic sensor is a proximity sensor which tests the presence of notches made to that end on the outer edge of the brake disc.
In such a case, it is typical that the electronic sensor for setting the speedometer on the instrument panel as well as the taximeter or chronotachograph into action, is one of the A.B.S. sensors already present on the vehicle. The electric pulses delivered by this sensor are then applied simultaneously to the A.B.S. braking system, to the speedometer on the instrument panel and to the taximeter or chronotachograph.
This raises a problem if it is desired to avoid the above-mentioned fraudulent action of the taximeter or chronotachograph, as, in that case, it is virtually no longer conceivable to connect the sensor in question to its user circuit by a mechanically shielded cable. This sensor is in fact located on the wheel and on a portion thereof which, although not rotating, is nonetheless subjected to sporadic displacements due to the articulated nature of this wheel.
Shielded cables which present a certain suppleness indeed exist on the market, but such suppleness is limited and in any case there would be a fairly fragile point of break at the point of connection of such a shielded cable on the sensor.
In addition, such a shielded cable would pass in the immediate proximity of the hydraulic conduits and the electric safety cables which are associated with the device braking the wheel in question. During the movements of articulation of the wheel, these hydraulic conduits and electric safety cables would then rub against the shielded cable, the movements of these different cables and conduits not being able to be strictly synchronous.
This would result in rapid wear of these hydraulic conduits and electric cables, and consequently a risk of piercing and rupturing of the circuit.
It is an object of the invention to overcome this drawback by proposing a process and a device for avoiding fraud on a taxi or a truck, even if the taxi or truck is equipped with an A.B.S. braking system.